1. Field of the Invention
This invention relates to an optical head having an imaging sensor and a tracking sensor adapted for use with a telescope having a drive system for correcting telescope position and more particularly relates to an imaging and tracking apparatus for a telescope wherein the apparatus includes an imaging CCD for imaging an object in a field of view and a tracking CCD for tracking a star or other celestial body which, in the preferred embodiment, is off axis to the field of view. The apparatus generates correction signals representative of the relative position of the telescope to a star being tracked. The correction signals are applied to the drive system to make corrections to the telescope position so as to maintain telescope alignment during imaging with an object in a field of view.
2. Description of the Prior Art
Long exposure imaging of the night sky is a technique that has long been used by astronomers to reveal detail beyond the limits of human vision. This technique has traditionally been performed by placing photographic film at the prime focus of a telescope, which has a clock drive to compensate for the rotation of the earth. The camera formed thusly is manually corrected to follow the stars while the earth rotates to acquire a sharp image.
Use of Charged Coupled Device (CCD) Camera systems in the field of astronomy and general imaging is well known in the art.
In astronomical imaging, the CCD sensor is ideal for imaging small, faint objects in a field of view. The CCD sensor is able to provide long term exposure of an object to develop an image which can be processed electronically. A CCD sensor based camera system has several advantages over a film imaging system. A CCD sensor based camera system provides faster imaging speeds, quantitative accuracy, ability to increase contrast and subtract sky background with few key strokes on a host computer, the ability to co-add multiple images without tedious darkroom operations, a wider spectral range and instant examination of the images at the telescope.
In astronomical imaging, the basic function of a sensor, such as a CCD, for example, is to convert incoming photons of light to electrons which are stored in the detector or sensor until the stored information is read out. Thus, a CCD sensor is able to produce data which a host computer can process and/or display as an image.
A telescope used for long exposure astronomical imaging of dim object, such as a galaxy, needs to have its position adjusted to compensate for the rotation of the earth relative to the object. Since most clock drive systems have some error, slight corrections to the drive rotation rate must be applied from time to time.
FIG. 1, labeled Prior Art, illustrates a typical CCD imaging camera system adapted for use with a astronomical telescope. The telescope, shown generally as 20, includes a main telescope, shown generally as 22 having a front lens system 24 and an eye piece 28. The telescope 20 is pivotally mounted around a support 34 to provide a means for rotatively adjusting the position of the telescope 20. A typical telescope includes an auxiliary or guiding telescope, shown generally as 30, which is rigidly mounted through a mount 34 to the main telescope 22. The auxiliary or guiding telescope 30 has an eye piece 38. The telescope 20 is mounted on a drive system 36 which has the ability to provide adjustments to this telescope in both right ascension and declination celestial coordinates.
CCD imaging cameras, which are well known in the art, provide the capability to either image an object in the field of view or to provide tracking of a star or other celestial body to develop correction signals for the telescope drive. The correcting signals are used to make corrections to the position of the telescope to maintain alignment of the telescope with an object in the field of view.
For example, the eye piece of the telescope 28 can be replaced by a film imaging camera attached thereto to record, on film, an object in a field of view. In such a case, a CCD tracking device could be utilized to provide correction signals to maintain telescope tracking.
Alternatively, a CCD imaging camera can be utilized as a means of recording or imaging an object in the field of view. This is illustrated in FIG. 1 which depicts the main telescope 22 having a CCD imaging camera 40 operatively replacing eye piece 28 to image an object in the field of view. Also, FIG. 1 illustrates that a CCD tracking device 44 can be operatively placed at the focus position 38 of the guiding telescope 30 to observe a star which can be utilized for developing position signals which are applied to the drive system 36 to make corrections to the telescope 20 to maintain alignment between the main telescope 22 and an object in the field of view.
In such a system, the CCD imaging camera 40 applies video signals, as depicted by arrow 46, to a driver/converter 50 to read out the imaging device. Similarly, a CCD tracking device 44 applies tracking signals, shown by arrow 48, to the driver/converter 50 to read out the tracking device. The driver/converter 50 then converts the video signals into digital signals which are transmitted, as depicted by arrow 52, to a host computer 54. The host computer 54 receives and processes the video signals from the CCD imaging camera 40 to develop an electronic image of an object in the field of view. Concurrently, the host computer 54 responses to the video signals generated by the CCD tracking device 44 to generate position correction signals which are applied, as shown by arrow 60, to a drive mechanism 36 which corrects the position of the telescope 20 to maintain alignment between the main telescope 22 including the CCD imaging camera 40 with an object in the field of view.
Alternatively, the video signals from the CCD tracking device 44 may be applied to a controller 46, shown by dashed lines and dashed box, such as for example an ST-4 controller sold by Santa Barbara Instrument Group, Santa Barbara, Calif. The output from the controller 46 is applied as shown by dashed line 42 to the drive base 36. The ST-4 controller includes separate CCD drivers, amplifiers and A/D converters. The ST-4 has a signal-to-noise ratio enabling the CCD camera to record faint objects.
The circuitry of the ST-4 controller is able to reduce dark current and increased charge accumulator per pixel before reaching saturation. These features increase the dynamic range of the CCD camera system used with the ST-4 controller.
The use of a CCD star tracker operatively coupled to the prime focus of either a guide telescope or the primary telescope is well known in the art. As mentioned before, one such known CCD star tracker is a Model ST-4 camera system offered for sale by Santa Barbara Instrument Group, Santa Barbara, Calif. When used as a star tracker, the Model ST-4 camera system generates control signals for guiding a telescope in right ascension and declination. In operation, a guide star is focused on the CCD detector, which consists of an array of elements, called pixels. The pixels are arranged in horizontal and vertical rows and, when illuminated with photons, packets of light energy from a star or object in a field of view, the photons are converted into electrons which are then converted into analog video signals for further processing.
In order to effectively use a CCD camera systems for astronomy, it is necessary for a telescope to have a drive mechanism and to utilize a tracking or autoguiding device to generate correcting signals which are applied to a host computer. The host computer generates correction signals to continually adjust the position of the telescope in response to relative movement between a tracking star and the telescope. Continual position adjustments are required in order to keep the object in a field of view in optical alignment with the telescope and the imaging device.
When the CCD star tracker is used to track a star, which is sometimes referred to as autoguiding, light energy from the star being tracked is present at a specific pixel within the CCD detector. A microcontroller detects a corresponding increase in the signal from that pixel. If relative movement between the telescope and a star occurs, light energy from the star being tracked will appear at a different pixel in the next exposure of the CCD sensor. The microcontroller then calculates how far the star has drifted and generates a control signal to correct the position of the telescope. The control signal iteration is a function of the star's positional error. The microcontroller can process an exposure, analyze the pixel of values and calculate the necessary telescope corrections in less than a second.
The Model ST-4 camera system can likewise be used as a CCD imaging camera with a telescope. When used as a CCD imaging camera, the Model ST-4 camera system is mounted at the focal position 28 of the telescope body 22 as depicted by imaging device 40. The Model ST-4 camera system is used in conjunction with a PC such as for example, an IBM XT/AT/PS/2 compatible PC or an Apple Macintosh. The Model ST-4 camera system will record and display images of objects that generally cannot be seen with the naked eye. The imaging CCD sensor within the Model ST-4 camera system converts incident photons into electrons, which are integrated within a pixel to form an image of an object in the field of view of the CCD sensor.
The Model ST-4 camera system is connected to a host computer having appropriate software to process the data.
The ST-4 was described well in an article entitled, "A Versatile CCD for Amateurs" by Dennis di Cicco which appeared at pages 250 through 255 in the September, 1990 Sky & Telescope (the "di Cicco Reference I"). It describes the ST-4 CCD camera system. SBIG provides an upgrade to the ST-4X, which has lower noise, but is not a stand-alone tracker. This was described in an article entitled "CCD Test Report: SBIG's new ST-4X CCD Camera System" by Dennis di Cicco in the Spring, 1994 CCD Astronomy (the "di Cicco Reference II"). This article described the tracking and imaging capabilities of the ST-4 camera. Specifically the di Cicco Reference II describes, in detail, the technical differences between the ST-4 CCD camera system and the ST-4X CCD camera system, the details of the operational software for the ST-4X CCD camera system and the details of the control program, generally known as CCDOPS, and the operating features thereof.
An ST-6 CCD imaging camera is offered for sale by the Santa Barbara Instrument Group, Santa Barbara, Calif. The ST-6 CCD imaging camera includes a track and accumulate mode which is intended to be a substitute for auto guiding, or star tracking. The ST-6 CCD camera system takes a series of user-specific "snapshots" (up to 64) which is disclosed by U.S. Ser. No. 07/964,775 filed Oct. 22, 1992 and entitled ELECTRONIC CAMERA WITH AUTOMATIC IMAGE TRACKING AND MULTI-FRAME REGISTRATION AND ACCUMULATION. The host computer automatically aligns each frame or "snapshot" on a specific reference star and then automatically adds the light energy from the object observed from the field of view to produce an image from the accumulated light energy.
The track and accumulate mode provides the advantage of compensating for errors which are introduced into the CCD sensor because of periodic errors in the telescope drive, the drift due to poor alignment or atmospheric refraction. The intended result is to provide a single image exposed for the same total duration as that of a mechanically adjusted telescope positioning system.
A description of the ST-6 CCD imaging camera is disclosed in the article entitled "ST-6 CCD Imaging Camera by Dennis di Cicco which appears at pages 395 through 400 in the October, 1992 issue of Sky and Telescope (the "di Cicco Reference III"). The di Cicco Reference III discloses, in detail, the technical aspects of the track and accumulate mode of the ST-6 CCD imaging camera. The di Cicco Reference III discloses that although no CCD camera can simultaneously take an exposure and guide a telescope, the track and accumulate feature has certain advantages for certain specific applications.
Originally, the typical CCD arrays available to professional astronomers were 512-by-512 pixel chips measuring about one half inch on a side; such an array when used with a telescope, provides a few arc minutes of field of view at the focus of most research telescopes. CCD arrays now known in the art are 2048 by 2048 pixel square arrays and can provide a large field of view of the sky for imaging.
It is also known in the art to provide a "Mini Mosaic" CCD camera which is built from an assembly of four (4) 2,048 pixel Square CCD arrays or other size arrays.
Prior Patents
A solid state sensing device having two photosensitive sections based on an interline transfer type CCD ("IT-CCD") is disclosed in U.S. Pat. No. 4,651,001. U.S. Pat. No. 4,651,001 discloses that the imaging sensing device has a photosensitivity corresponding to two different light wavelength ranges. The first radiation photosensing section used for sensing infrared radiation is formed on a substrate. The substrate is covered by insulative layers and a transparent electrode is formed on the insulative layer to serve as a pixel electrode. A photoconductive layer or amorphous silicon layer serves as a second photosensing section on the structure. The amorphous silicon layer is photosensitive to a predetermined visible light wavelength range so as to be sensitive to visible radiation. Thus, the first photosensing section senses only infrared radiation and the second photosensing section senses only visible radiation. The two photosensing sections are stacked on a substrate in a two-layer structure.
U.S. Pat. No. 4,651,001, although disclosing two separate distinct photosensing sections performing two functions, do not disclose, teach or suggest that the two photosensing sections or sensors can be positioned in a spaced adjacent arrangement. In the structure provided in U.S. Pat. No. 4,651,011, a Schottky diode is used at the infrared radiation sensor. The amorphous silicon layer is used as the visible light sensor. It is necessary to maintain a large temperature gradient between the Schottky diode and the amorphous silicon layer and each layer must be cooled to two different temperature levels.
U.S. Pat. No. 4,651,001 does not disclose, teach or suggest the use of two separate sensors for tracking and imaging for use in astronomical imaging and processing of an object in a field of view.
It is also known in the art to utilize multiple CCD arrays in a system adapted for tracking either a star or a missile. Typical of such CCD arrays are those disclosed in U.S. Pat. Nos. 5,300,781; 5,066,860 and 5,012,081.
Each of U.S. Pat. Nos. 5,300,781; 5,066,860 and 5,012,081 utilize the CCD arrays as star trackers or optical sensors for detecting a guide star and utilize the same for controlling directions of a missile, satellite or space vehicle relative to the star.
It is also known in the art to utilize a single CCD imaging systems which are mounted on telescopes, used to track stars or air borne objects such a helicopters. Typical of such systems are those disclosed in U.S. Pat. Nos. 5,341,435; 5,260,557; 5,177,686; 5,162,861; 4,944,587 and 4,388,646.
U.S. Pat. No. 4,939,369 discloses an imaging and tracking sensor designed for providing a multi function imaging and tracking capability. U.S. Pat. No. 4,939,369 discloses that the multi function imaging and tracking device is in the form of a Schottky diode array lying in a first plane to perform a tracking and acquisition function. A second Schottky diode array lying in a second plane performs an imaging function. The first array is a low resolution. high speed array and the second array is a high resolution, low speed array. U.S. Pat. No. 4,939,369 discloses that the arrays are formed in a stack relationship wherein the first array responds to short wave length radiation of interest which is focused on a tracking array or a long wave length radiation of interest is focused on the imaging array 24. Further, the imaging tracking sensor is responsive to a reflected beam such, as for example, a laser beam. The laser beam is projected upon a target and a portion of the laser beam is reflected from the target to the tracking and imaging structure for detection and processing.
The U.S. Pat. No. 4,939,369 discloses that the structure can also be utilized to provide for monitoring of visible and infrared radiation rather than for a tracking and imaging application.
U.S. Pat. No. 4,939,369 deals with a different problem, mainly tracking and imaging on the same object. It does not reveal the technique of guiding on an off axis object with the second array, such as is possible in astronomical imaging. In fact, the concept of tracking on the object being imaged works poorly in astronomy since such a technique requires the light to be divided between two sensors, with the associated loss of signal, and the object being imaged is often extremely dim.